In the course of general development in power economy, alcohol has gained increasing importance as a motor fuel or motor fuel additive. Alcohol for such purposes can be made from cellulose or cellulose-containing biomass in two stages by firstly hydrolyzing cellulose to form sugar and then fermenting it to form ethanol. While the fermentation of sugar to form ethanol is technically well controlled, the hydrolysis of cellulose is still the critical process step which determines the overall economy of the method.
There are shortcomings in the known methods of acid-catalysed hydrolysis of cellulose, which are based essentially on the Bergius and Scholler processes (DE-C-577 850), particularly in that the energy content of the alcohol produced is frequently lower than the energy which is required to operate the entire plant and which is required to operate the entire plant and which must be provided particularly in the form of heating steam and electric current.
Percolator-type fixed bed reactors for semicontinuous hydrolysis are known, for example, from DE-A-15 67 350 and DE-A-15 16 335, batch diluted sulphuric acid running over a fixed bed of wood chips, and cellulose being converted into glucose with a yield of approximately 50% at a hydrolysis temperature of 120.degree. to 145.degree. C. and a sojourn time of 15 to 60 minutes. In addition to the relatively adverse glucose yield, a high specific expenditure of energy is required in this case.
An essential theoritical attempt towards improvements results from the publication of Hans E. Grethlein in the journal "Biotechnology and Bioengineering", Vol. II (1978), Pages 503 to 525 "Comparison of the Economics of Acid and Enzymatic Hydrolysis of News-print". In this publication it was stipulated that a high output of glucose relative to the ALPHA cellulose used is achieved if the hydrolysis temperatures are increased by 250.degree. C. up to 300.degree. C. at pressures of 40 to 90 bars, and if dilute sulphuric acid is used with a concentration of up to 2.0% and the hydrolysis time is extremely short.
Starting from this publication, U.S. Pat. No. 4,708,746 disclosed a method of the type as defined in which high outputs of fermentable sugar relative to the cellulose-containing substrate used should be achieved with a low expenditure of energy. With this method the steam which is extracted from the reactor during the removal of pressure was stored in accumulator with different pressure levels and used again for a partial pressure build-up in a subsequent hydrolysis cycle. Although the method of energy recuperation is initially advantageous, the expenditure or external energy and the output were still unsatifactory.